With reference to FIG. 1, a typical turbocharger 101 having a radial turbine includes a turbocharger housing and a rotor configured to rotate within the turbocharger housing along an axis of rotor rotation 103 on thrust bearings and two sets of journal bearings (one for each respective rotor wheel), or alternatively, other similarly supportive bearings. The turbocharger housing includes a turbine housing 105, a compressor housing 107, and a bearing housing 109 (i.e., a center housing that contains the bearings) that connects the turbine housing to the compressor housing. The rotor includes a turbine wheel 111 located substantially within the turbine housing, a compressor wheel 113 located substantially within the compressor housing, and a shaft 115 extending along the axis of rotor rotation, through the bearing housing, to connect the turbine wheel to the compressor wheel.
The turbine housing 105 and turbine wheel 111 form a turbine configured to circumferentially receive a high-pressure and high-temperature exhaust gas stream 121 from an engine, e.g., from an exhaust manifold 123 of an internal combustion engine 125. The turbine wheel (and thus the rotor) is driven in rotation around the axis of rotor rotation 103 by the high-pressure and high-temperature exhaust gas stream, which becomes a lower-pressure and lower-temperature exhaust gas stream 127 and is axially released into an exhaust system (not shown).
The compressor housing 107 and compressor wheel 113 form a compressor stage. The compressor wheel, being driven in rotation by the exhaust-gas driven turbine wheel 111, is configured to compress axially received input air (e.g., ambient air 131, or already-pressurized air from a previous-stage in a multi-stage compressor) into a pressurized air stream 133 that is ejected circumferentially from the compressor. Due to the compression process, the pressurized air stream is characterized by an increased temperature over that of the input air.
Optionally, the pressurized air stream may be channeled through a convectively cooled charge air cooler 135 configured to dissipate heat from the pressurized air stream, increasing its density. The resulting cooled and pressurized output air stream 137 is channeled into an intake manifold 139 on the internal combustion engine, or alternatively, into a subsequent-stage, in-series compressor. The operation of the system is controlled by an ECU 151 (engine control unit) that connects to the remainder of the system via communication connections 153.
U.S. Pat. No. 4,850,820, dated Jul. 25, 1989, which is incorporated herein by reference for all purposes, discloses a turbocharger similar to that of FIG. 1, but which has an axial turbine. The axial turbine inherently has a lower moment of inertia, reducing the amount of energy required to accelerate the turbine. As can be seen in FIG. 2, the turbine has a scroll that circumferentially receives exhaust gas at the radius of the turbine blades and (with reference to FIG. 1) axially restricts the flow to transition it to axial flow. It thus impacts the leading edge of the turbine blades in a generally axial direction (with reference to col. 2).
For many turbine sizes of interest, axial turbines typically operate at higher mass flows and lower expansion ratios than comparable radial turbines. While conventional axial turbines generally offer a lower inertia, albeit with some loss of efficiency and performance, they suffer from an inability to be efficiently manufactured in the small sizes usable with many modern internal combustion engines. This is, e.g., due to the exceptionally tight tolerances that would be required, due to aerodynamic limitations, and/or due to dimensional limitations on creating small cast parts. Axial turbines also lack the ability to perform well at higher expansion ratios, such as are typically needed due to the pulsing nature of the exhaust of an internal combustion engine. Furthermore, conventional axial turbines have a significant change in static pressure across the blades, causing significant thrust loads on the thrust bearings of the rotor, and potentially causing blowby.
In some conventional turbochargers the turbines and compressors are configured to exert axial loads in opposite directions so as to lessen the average axial loads that must be carried by the bearings. Nevertheless, the axial loads from the turbines and compressors do not vary evenly with one another and may be at significantly different levels, so the thrust bearings must be designed for the largest load condition that may occur during turbocharger use. Bearings configured to support high axial loads waste more energy than comparable low-load bearings, and thus turbochargers that must support higher axial loads lose more energy to their bearings.
Accordingly, there has existed a need for a turbocharger turbine having a low moment of inertia, and characterized by a small size that does not require exceptionally tight tolerances, while having reasonable efficiency both at both lower and higher expansion ratios, and smaller axial loads. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.